1. Field of the Invention
The present invention relates to a vehicular steering system having a steering mechanism for steering the steerable road wheels of a vehicle in accordance with a value that is obtained by mechanically adding the steering wheel angle of a steering wheel installed on the vehicle and an auxiliary steering angle due to an electrically controlled auxiliary steering angle superposition mechanism to each other. In particular, the invention relates to such a system that performs steering (intervention steering) to correct the steering operation of the steering wheel by a driver, and at the same time changes a transmission characteristic between the steering angle of the steering wheel operated by the driver and the steered angle of the steerable road wheels.
2. Description of the Related Art
Conventionally, there has been known a vehicular steering system in which an auxiliary steering angle superposition mechanism and a steering mechanism are interposed between a steering wheel and steerable road wheels of a vehicle, so that the steerable road wheels are steered by superposing an amount of auxiliary steering by an electric motor in the auxiliary steering angle superposition mechanism on an amount of steering of the steering wheel operated by a driver. A planetary gear mechanism, a differential gear mechanism or the like is used as the auxiliary steering angle superposition mechanism. Such a vehicular steering system changes a transmission characteristic of the steering angle of the steering wheel to the steered angle of the steerable road wheels in accordance with the traveling condition of a vehicle.
For example, a conventional vehicular steering control apparatus as described in a first patent document (Japanese patent No. 3518590) is provided with a transmission ratio variable mechanism that changes a transmission ratio (transmission characteristic) between the steering angle of the steering wheel (steering wheel angle) and the steered angle of steered road wheels (steerable road wheels). In addition, the conventional vehicular steering control apparatus is also provided with a steering angle detection section that detects the steering angle of the steering wheel as a relative angle, a driving section that drives the transmission ratio variable mechanism to rotate, an operation angle detection section that detects the operation angle of the driving section (auxiliary steering angle) as an absolute angle, and a control section that drives and controls the driving section based on the transmission ratio set in accordance with the traveling state of the vehicle and the steering angle detected by the steering angle detection section. The control section is provided with an estimation section that estimates an absolute steering angle of the steering wheel based on the detection result of the operation angle detection section and the transmission ratio set in the transmission ratio variable mechanism (see, for example, the first patent document).
Further, another conventional vehicular steering control apparatus as described in a second patent document (Japanese patent No. 3489112) is provided with a transmission ratio variable mechanism that can change a transmission ratio (transmission characteristic) between the steering angle of a steering wheel (steering wheel angle) and the steered angle of steered road wheels (steerable road wheels). Also, this conventional vehicular steering control apparatus is provided with an actuator for driving the transmission ratio variable mechanism and a control section for controlling the operation of the actuator. The control section is provided with a control amount setting section that temporarily sets a target steered angle value in the form of a target value of the steered angle and at the same time sets a main control amount for the actuator, a first correction section that sets, upon occurrence of a phase deviation in which a neutral position of the steering wheel and a neutral position of the steered road wheels are different from each other, a first correction amount to correct the temporarily set target steered angle value so as to leave the phase deviation, and a second correction section that sets a second correction amount to decrease the first correction amount so as to reduce the phase deviation. The control amount setting section calculates the target steered angle value in the form of the target value of the steered angle based on a deviation between the temporarily set target steered angle value and the first correction amount decreased by the second correction section, and sets the main control amount for the actuator based on a deviation between the calculated target steered angle value and the steered angle of the steered road wheels (see, for example, the second patent document).
In the conventional apparatus of the above-mentioned first patent document, the transmission characteristic of the steering wheel angle to the steered angle of the steerable road wheels is set based on the traveling condition of the vehicle including the vehicle speed, the steering speed of the steering wheel, etc., and the target steered angle is calculated based on the steering wheel angle and the transmission characteristic. In addition, a target auxiliary steering angle is calculated based on a characteristic that is decided from the target steered angle and the mechanical construction of the auxiliary steering angle superposition mechanism.
A rotary encoder or the like is used as a sensor for detecting the steering wheel angle or the steered angle, as shown in the above-mentioned first patent document. The rotary encoder calculates an angle by outputting two-phase pulse signals comprising a combination of “0” and “1”, and counting these pulse signals, and hence, can not detect an absolute angle upon activation thereof. Accordingly, the conventional apparatus of the above-mentioned first patent document starts control by estimating the steering wheel angle and the absolute angle of the steered angle upon activation thereof based on the auxiliary steering angle, which is detected as an absolute angle, stored in memory and does not change during the time when the apparatus is stopped, and the transmission characteristic.
Here, note that when the steering wheel is steered while the apparatus is stopped, the steering wheel angle estimated from the auxiliary steering angle and the transmission characteristic is different from the actual steering wheel angle and hence becomes an incorrect value. In addition, the steered angle calculated based on the incorrectly estimated steering wheel angle becomes an incorrect value, as a result of which the neutral position of the steering wheel and the neutral positions of the steered road wheels are set to positions different from their actual neutral positions, respectively.
At this time, the auxiliary steering angle to be superposed by the auxiliary steering angle superposition mechanism is set based on the incorrectly estimated steering wheel angle, and the auxiliary steering angle superposition mechanism is driven to operate in accordance with the auxiliary steering angle thus incorrectly set. As a consequence, in case where the transmission characteristic includes a nonlinear characteristic, a right and left difference is generated in the steered angle characteristic of the steerable road wheels. Here, the meaning that the transmission characteristic indicates a nonlinear characteristic is that the ratio of the change of the steered angle with respect to the change of the steering wheel angle in the vicinity of the neutral positions of the steerable road wheels and the ratio of the change of the steered angle with respect to the change of the steering wheel angle at a location away from the neutral positions of the steerable road wheels are different from each other.
Accordingly, an uncomfortable feeling due to the deviation of the neutral position of the steering wheel or an uncomfortable feeling due to the right and left difference of the vehicle behavior would be caused to the driver of the vehicle, thus giving rise to a problem that there occurs a mismatch or discrepancy between driver's steering feeling and the actual vehicle behavior.
In order to solve such a problem, the conventional apparatus of the above-mentioned first patent document detects the speeds of the right and left steerable wheels, respectively, during travel of the vehicle, and estimates the steered angle based on the detected road wheel speeds. In addition, the steering wheel angle and the steered angle are corrected based on a deviation between the estimated steered angle and the detected steered angle.
Moreover, when the neutral position of the steering wheel and the neutral positions of the steerable road wheels are different from each other, i.e., when the target steered angle value, which is calculated based on the steering wheel angle and the transmission characteristic, and the detected steered angle are different from each other, the conventional apparatus of the above-mentioned second patent document corrects the target steered angle value by using a correction amount corresponding to a deviation between the target steered angle value and the detected steered angle.
Further, by changing this correction amount toward “0”, abrupt steering of the steerable road wheels is suppressed, and the neutral position of the steering wheel and the neutral positions of the steerable road wheels are made to coincide with each other.
In the vehicular control apparatus described in the above-mentioned first patent document, the steering wheel angle and the steered angle are corrected based on the deviation between the estimated steered angle and the detected steered angle, but there is a problem that the right and left difference of the vehicle behavior continues until the above-mentioned correction is finished.
Also, the vehicular control apparatus described in the above-mentioned second patent document, the target steered angle value is corrected by using a correction amount corresponding to a deviation between the target steered angle value and the detected steered angle. However, in case where the target steered angle value is corrected, the steerable road wheels are deviated an angle corresponding to a correction amount from their neutral positions, so there is a problem that a right and left difference in the vehicle behavior occurs.